IANAL, but my guess is that it’s not economically feasible ot run negative ads. Either:
1. The laws governing “libel” or “false claims” are much stricter when dealing with products, copyrights, and corporations
and/or (more likely)
2. You’re talking about a much longer timeframe when you’re advertising products or brands… it’s infeasible for a candidate to take legal recourse against an attack ad (because by the time you even filed the paperwork the issue would be moot), whereas a lot of corporate lawyers at Pepsi would enjoy huge Christmas bonuses should Coke run an attack ad. Why take the economic risk of spending half a million on an ad and then risk additional litigation costs?
…which brings me to:
3. It’s a different market. I think part of the reason attack ads work so well in the political arena is because there are a small handful of clearly defined “brands” in the market—and usually tearing down one candidate will cause people to choose a different one. The only time an attack ad would work on the corporate side is if there are only a few clearly distinguishable brands in a space, and even then I don’t think that translates into a broader market—if you make people think “Wow, Pepsi stinks”, it doesn’t translate into them buying a coke.
The closest you’re going to get to attack ads on the corporate side are the Apple ads.